1. Field of the Invention
This invention relates to circuit interrupters and, more particularly, it pertains to a two-pressure type circuit breaker for extinguishing arcs occurring between separating contacts.
2. Description of the Prior Art
It is well known that the performance of an arc-extinguishing gas, such as sulfur hexafluoride, as a current interrupting medium increases with the difference between the upstream and downstream pressures; that is, the pressure drop through the interrupter nozzle. Two gas handling methods have evolved, the puffer and the two-pressure systems, to obtain this pressure drop. In the puffer type circuit breaker the upstream gas is compressed by a piston which operates during the opening stroke. However, in the two-pressure system, the upstream gas is compressed ahead of time and discharged during the opening stroke by means of a valve.
In the puffer type circuit breaker the piston is physically connected to the movable electrode. The device is therefore limited to the compression ratio that can be produced by the motion of the piston. The ratio is normally quite small, but there are circumstances when the pressure ratio gets very high during arcing. A blow-back situation can occur during the high current pressure of the cycle to cause clogging of the orifice and stopping of the mechanism. The additional pressure difference developed during this period is available for extinguishing the arc at the next current zero, provided the gas that was pushed upstream is cool enough. It has been found that metallic coolers are important in the upstream region; pressures of 25-30 atmospheres with a primary filling of 4 atmospheres have been reported.
In the two-pressure system, the pressure drop is limited by the boiling point of SF.sub.6 which allows about 16 atmospheres for the upstream pressure, if the reservoir is maintained at room temperature. The downstream pressure can be essentially atmospheric. It has been found however that every effort must be made to prevent severe clogging.